As a configuration of an AC-AC power converter, an indirect-type AC power converter is generally used. In an indirect-type AC power converter, a commercial AC is converted into a DC by a rectifier circuit and, additionally through a smoothing circuit, power is supplied to a voltage-source converter, so that an AC output is obtained from the voltage-source converter.
On the other hand, a direct-type AC power converter, such as a matrix converter, which performs an AC-AC conversion without using a smoothing circuit is also known. The matrix converter performs the AC-AC conversion by using a commutation in a group of switching elements.
However, as introduced in Japanese Patent Application Laid-Open No. 2004-222338 and Rie Itoh and Isao Takahashi, “Decoupling Control of Input and Output Reactive Power of the Matrix Converter”, IEEJ Technical Meeting on Semiconductor Power Converter, SPC-01-121, IEA-01-64, the matrix converter can, even though actually having no DC link, control a commutation in the group of switching elements based on an operation of a configuration in which a virtual AC-DC converter and a virtual DC-AC converter are coupled with each other through a virtual DC link having no smoothing circuit. This matrix converter is called a direct matrix converter.
As introduced in Lixiang Wei, Thomas. A Lipo, “A Novel Matrix Converter Topology With Simple Commutation”, IEEE IAS 2001, vol. 3, 2001, pp 1749-1754, a configuration in which an AC-DC converter and a DC-AC converter are coupled with each other through a DC link having no smoothing circuit has also been proposed as an embodiment of the matrix converter. This matrix converter is called an indirect matrix converter.
Furthermore, Japanese Patent No. 4067021 introduces an inverter in which even though a capacitor is provided in a DC link, the capacitance of the capacitor is selected and set to be so small as not to function as a smoothing circuit. This technique presupposes that a voltage of the DC link ripples.
Therefore, in the present application, irrespective of whether or not a DC link is formally provided or whether or not a capacitor is formally provided, a circuit performing an AC power conversion without interposition of a real smoothing circuit is referred to as a direct-type AC power converter.
In the direct-type AC power converter, a capacitor and a reactor which have a large size and smooth a voltage ripple caused by a commercial frequency are not required, and therefore downsizing of the converter can be expected. Thus, in recent years, the direct-type AC power converter is attracting attention as a next-generation power converter.
The need to process ripple energy in the direct-type AC power converter is mentioned in “Trend of Direct AC/AC Converters and Related Technologies”, Technical Report No. 998 of the Institute of Electrical Engineers of Japan, pp 24-25.